Along with the development of personal computers, cell phones, and other portable devices in recent years, the demand for secondary batteries as a power source therefor has increased dramatically. Electrolytes comprising an organic solvent or other liquid state (liquid electrolytes) have been widely used as a medium for ion transfer in secondary batteries utilized in such applications. However, problems such as leakage of the liquid electrolyte, fire, and explosion can occur in batteries using such a liquid electrolyte.
From the standpoint of ensuring the intrinsic safety of the secondary battery, the use of a solid electrolyte in place of the liquid electrolyte has progressed, as well as the development of an all-solid-state secondary battery in which all other battery components are also solid. Because the electrolyte is a sintered ceramic, this kind of all-solid-state secondary battery is advantageous since there is no fear of fire or leakage, and problems such as deterioration of battery performance due to corrosion are unlikely to occur. Among these, an all-solid-state lithium secondary battery utilizing lithium metal in the electrodes is believed to be a secondary battery that can easily provide a high energy density.
To improve battery properties in the secondary battery, the point therefor becomes widening the difference in potential between the materials used in the cathode and anode, and increasing the specific capacity of the materials used in the poles. With respect to the anode material in particular, it has been found that using lithium metal and lithium alloys imparts a large improvement in the properties thereof. However, because a lithium metal precipitation phenomenon known as dendrite crystallization occurs in association with a lithium intercalation reaction, the dendritic lithium metal can penetrate the separator in a battery utilizing a liquid electrolyte in the electrolyte member and cause a short inside the battery; thus lithium metal could not be used due to safety problems. It is presumed that the all-solid-state battery in which the electrolytic member is formed from a solid can be used safely because the precipitate cannot penetrate the solid electrolyte. However, lithium metal not only has the poorest electrical potential, but it is also highly reactive, so a useable solid electrolyte has not been discovered heretofore.
It has recently been reported that Li7La3Zr2O12 (hereinafter LLZ), a garnet-type ceramic material, has excellent lithium resistance and can be utilized as a solid electrolyte in an all-solid-state lithium secondary battery (Non Patent Literature 1).
Non Patent Literature 1
Ramaswamy Murugan et al., Agnew. Chem. Int., Ed. 2007, 46, 1 to 5.